Small vehicles or self-propelled lawn and garden implements, such as snow throwers, lawn mowers, lawn and garden tractors, and the like, include an energy source, such as an internal combustion engine, which provides power for rotatably driving an axle which is coupled to rotatably driven wheels. Typically, the energy source operates at a single, rotary mechanical speed. Yet, for practical reasons, the axle needs to be able to be rotatably driven at a variety of speeds in forward and reverse, and to be provided with a neutral mode. Accordingly, such vehicles may incorporate a transmission or transaxle which is used to convert the single speed, rotary mechanical motion of the energy source into a variety of output speeds.
Generally, a transmission comprises: a transmission input shaft which is operationally coupled to the energy source; a transmission output shaft, e.g., an axle in the case of a transaxle, which is operationally coupled to the elements, e.g., wheels, which are to be rotatably driven; and transmission componentry which operationally couples the transmission's input and output shafts. It is the transmission componentry which converts the single speed, rotary mechanical motion received from the energy source into a variety of output speeds for rotatably driving the output shaft.
A variety of forms of transmission componentry have been known in the art. For example, one embodiment of a transaxle which has been known in the art incorporates a clutch pedal and a gear shifting mechanism which are used to adjust the speed of the output shaft. Such transaxles require that the clutch be depressed in order to accomplish a change in direction and/or speed. In some instances, the vehicle or implement in which the transmission is installed must be completely stopped before a direction or speed change can be accomplished.
As an improvement to clutch controlled transmissions, various types of clutchless, variable speed transaxles have been developed which control output speed through a single lever. In a typical mode of operation, the lever is moved forward to move the vehicle in the forward direction or pulled backward to move the vehicle in the reverse direction. The farther forward or backward the lever is displaced, the faster the vehicle travels in the corresponding direction.
One form of variable speed transaxle now in use includes hydrostatic componentry in which a variable fluid drive transfers variable speed rotary motion to the output shaft through a mechanical gear train. Although reliable, such transmissions tend to be relatively heavy and large in size, require greater amounts of horsepower to operate which would otherwise be available for mower or other implement operation, and are relatively expensive to manufacture and install due to complex componentry.
Another form of transaxle includes friction drive componentry in which a traction wheel engages a drive disk. The traction wheel is moved radially across the center of rotation of the drive disk to vary output speed and/or direction. Such componentry, however, is unable to transfer large torque and, therefore, tends to slip within certain speed and/or load ranges. Moreover, such systems may require frequent servicing due to a lack of reliability.
Clutchless transmissions which control output speed entirely through mechanical means have also been proposed. A representative transaxle embodiment of this form is described, for example, in U.S. Pat. No. 4,726,256 (Von Kaler). This transaxle uses dual drive power input shafts for connection to a power source. The dual input shafts are parallel, rotate in the same direction, and are coupled to a planetary gear system incorporating a sun gear, planet gears, and a ring gear. The relative rotational speeds between the two input shafts cause the planet gears to revolve clockwise, counterclockwise, or remain motionless relative to the axis of the sun gear. The speed of revolution of the planet gears is transferred to the output shaft through an output gear assembly to which the planet gears are coupled. Thus, varying the relative rotational speed of the two input shafts is used to control the revolution speed and direction of the planet gears, and hence the rotational speed and direction of the output shaft.
The transaxle of U.S. Pat. No. 4,726,256 thus provides a variable speed transaxle in which dual transaxle input shafts are coupled to the transaxle output shaft by a mechanical drive train incorporating a planetary gear system. Such a transaxle offers the potential for highly efficient power transfer from the input shafts to the output shaft. Nonetheless, the need to always drive two different input shafts may adversely affect the maximum efficiency which could be achieved by the transaxle. Further, the transaxle uses mechanical means to provide a braking action for controlling engine speed, which increases the weight and mechanical complexity of the transaxle.
Different types of pumps may be incorporated into vehicle drive systems. One known type of pump includes an externally toothed pinion gear connected to its input shaft, and the pinion gear meshes with an epicyclic gear mechanism. As the fluid path through the pump changes, the rotation speed of the pinion gear and epicyclic gear mechanism changes. However, incorporation of this separate pump into a transaxle may increase the complexity and manufacturing costs of such a transaxle, and further may frustrate attempts to provide a sufficiently compact and conveniently sized transaxle for certain applications.
Accordingly, there remains a need in the art for a reliable, simple, economic, relatively compact, clutchless, rugged, durable, variable speed transaxle which provides forward, reverse, and/or neutral speed operation.